NGC1097 é uma galáxia espiral a uma distância de 14.5 Mpc, com um núcleo LINER (do inglês Low-Ionization Nuclear Emission Region) de baixa luminosidade, a partir do qual foi descoberta em 1991 emissão larga de duplo pico nas linhas de Balmer do hidrogênio. O perfil destas linhas mostrou-se, em trabalho anterior, ser bem reproduzido por um modelo de disco de gás em rotação (disco de acreção), com velocidades de até 10000 km s¡1. Neste trabalho modelamos de forma autoconsistente o espectro contínuo de energia do núcleo LINER obtido a partir de observações recentes com o Telescópio de Raios X Chandra e com o Telescópio Espacial Hubble, bem como dados de arquivo na região dos comprimentos de onda de rádio e infravermelho. Além disso, utilizamos modelos de fotoionização para modelar o fluxo das linhas de emissão largas com duplo pico. O contínuo é modelado como tendo origem num escoamento acretivo de plasma ao redor do buraco negro central. A parte interna do escoamento mais próxima do buraco negro (R < 225RS, onde RS é o raio de Schwartzschild) é opticamente fina, dominada por advecção (chamada ADAF) e a parte externa (R > 225RS) consiste num disco de acreção geometricamente fino, opticamente espesso. A parte interna do escoamento (ADAF) é responsável por emissão que se estende da região espectral de rádio até raios X do contínuo observado, enquanto a parte externa (disco fino) é responsável por uma fraca emissão contínua no óptico - ultravioleta. Modelamos as intensidades das linhas de baixa ionização largas com duplo pico observadas com o telescópio Hubble como emissão da atmosfera do disco fino fotoionizada pelo contínuo ionizante emitido do ADAF, usando o programa Cloudy. Levando em conta que usamos uma prescrição simples da iluminação do disco, obtivemos uma boa concordância entre os resultados da simulação e as observações das linhas. É a primeira vez que é realizada uma modelagem autoconsistente como esta, no sentido de que os parâmetros utilizados na modelagem do contínuo e do fluxo das linhas de emissão são os mesmos derivados da modelagem dos perfis das linhas de emissão com duplo pico em trabalhos anteriores. Com a modelagem detalhada do contínuo de NGC1097 apresentada neste trabalho somada à modelagem da variabilidade temporal do perfil da linha H alfa realizada em trabalhos anteriores, fortalecemos o cenário ADAF + disco fino para o escoamento acretivo como origem das propriedades características dos AGNs emissores de linhas largas com duplo pico. Além disso, este trabalho reforça a hipótese de que as baixas luminosidades dos LLAGNs (Low-Luminosity Active Galactic Nuclei) são uma consequência da existência de escoamentos de baixa eficiência radiativa nos seus núcleos. / NGC1097 is a spiral galaxy at a distance of 14.5 Mpc which harbors a low-luminosity LINER (Low-Ionization Nuclear Emission Region) nucleus. In 1991 this galaxy displayed the abrupt appearance of broad double-peaked Balmer lines, and in subsequent work the double-peaked H® line profile was modelled as emission from a disk of rotating gas (accretion disk), with velocities up to 10000 km s¡1. In this work we model in a self-consistent way the continuum energy spectrum of the LINER nucleus of NGC1097, obtained through recent observations with the X-ray telescope Chandra and the Hubble Space Telescope, as well as data in the wavelength region of radio. Furthermore, we used a photoionization code to model the flux of the broad double-peaked emission lines. The continuum is modelled as being originated in an accretion flow of plasma around the central black hole. The inner part of the flow closer to the black hole (R < 225RS, where RS is the Schwartzschild radius) is optically thin, geometrically thick, advection- dominated (named ADAF) and the outer portion consists of a geometrically thick, optically thin accretion disk. The ADAF is responsible for continuum emission that extends from radio to X-rays, while the thin disk is responsible for a weak continuum emission in the optical to ultraviolet. Using the code Cloudy, we modelled the intensities of the broad, double-peaked, low-ionization emission lines observed with the Hubble telescope, as emission from the atmosphere of the thin disk photoionized by the ionizing continuum emitted by the ADAF. Considering that we adopted a simplified prescription for the illumination of the accretion disk, we obtained a good agreement between the results of the simulation and the observations of the lines. It is the first time that such self-consistent modelling is done, in the sense that the parameters used in the modelling of the continuum and the flux of the double-peaked emission lines are the same as those derived in the modelling of double-peaked emission-line profiles in previous works. With the detailed continuum modelling of NGC1097 presented in this work added to the modelling of the temporal variability of the H® emission-line profile accomplished in previous works, we strengthen the scenario in which an ADAF + thin disk is responsible for the characteristic properties of active galactic nuclei (AGN) emitting \disklike" lines. Furthermore, this work furthers the hypothesis that the low luminosities of low-luminosity AGN are a consequence of the presence of radiatively ineffcient accretion flows in their nuclei.

NGC1097 é uma galáxia espiral a uma distância de 14.5 Mpc, com um núcleo LINER (do inglês Low-Ionization Nuclear Emission Region) de baixa luminosidade, a partir do qual foi descoberta em 1991 emissão larga de duplo pico nas linhas de Balmer do hidrogênio. O perfil destas linhas mostrou-se, em trabalho anterior, ser bem reproduzido por um modelo de disco de gás em rotação (disco de acreção), com velocidades de até 10000 km s¡1. Neste trabalho modelamos de forma autoconsistente o espectro contínuo de energia do núcleo LINER obtido a partir de observações recentes com o Telescópio de Raios X Chandra e com o Telescópio Espacial Hubble, bem como dados de arquivo na região dos comprimentos de onda de rádio e infravermelho. Além disso, utilizamos modelos de fotoionização para modelar o fluxo das linhas de emissão largas com duplo pico. O contínuo é modelado como tendo origem num escoamento acretivo de plasma ao redor do buraco negro central. A parte interna do escoamento mais próxima do buraco negro (R < 225RS, onde RS é o raio de Schwartzschild) é opticamente fina, dominada por advecção (chamada ADAF) e a parte externa (R > 225RS) consiste num disco de acreção geometricamente fino, opticamente espesso. A parte interna do escoamento (ADAF) é responsável por emissão que se estende da região espectral de rádio até raios X do contínuo observado, enquanto a parte externa (disco fino) é responsável por uma fraca emissão contínua no óptico - ultravioleta. Modelamos as intensidades das linhas de baixa ionização largas com duplo pico observadas com o telescópio Hubble como emissão da atmosfera do disco fino fotoionizada pelo contínuo ionizante emitido do ADAF, usando o programa Cloudy. Levando em conta que usamos uma prescrição simples da iluminação do disco, obtivemos uma boa concordância entre os resultados da simulação e as observações das linhas. É a primeira vez que é realizada uma modelagem autoconsistente como esta, no sentido de que os parâmetros utilizados na modelagem do contínuo e do fluxo das linhas de emissão são os mesmos derivados da modelagem dos perfis das linhas de emissão com duplo pico em trabalhos anteriores. Com a modelagem detalhada do contínuo de NGC1097 apresentada neste trabalho somada à modelagem da variabilidade temporal do perfil da linha H alfa realizada em trabalhos anteriores, fortalecemos o cenário ADAF + disco fino para o escoamento acretivo como origem das propriedades características dos AGNs emissores de linhas largas com duplo pico. Além disso, este trabalho reforça a hipótese de que as baixas luminosidades dos LLAGNs (Low-Luminosity Active Galactic Nuclei) são uma consequência da existência de escoamentos de baixa eficiência radiativa nos seus núcleos. / NGC1097 is a spiral galaxy at a distance of 14.5 Mpc which harbors a low-luminosity LINER (Low-Ionization Nuclear Emission Region) nucleus. In 1991 this galaxy displayed the abrupt appearance of broad double-peaked Balmer lines, and in subsequent work the double-peaked H® line profile was modelled as emission from a disk of rotating gas (accretion disk), with velocities up to 10000 km s¡1. In this work we model in a self-consistent way the continuum energy spectrum of the LINER nucleus of NGC1097, obtained through recent observations with the X-ray telescope Chandra and the Hubble Space Telescope, as well as data in the wavelength region of radio. Furthermore, we used a photoionization code to model the flux of the broad double-peaked emission lines. The continuum is modelled as being originated in an accretion flow of plasma around the central black hole. The inner part of the flow closer to the black hole (R < 225RS, where RS is the Schwartzschild radius) is optically thin, geometrically thick, advection- dominated (named ADAF) and the outer portion consists of a geometrically thick, optically thin accretion disk. The ADAF is responsible for continuum emission that extends from radio to X-rays, while the thin disk is responsible for a weak continuum emission in the optical to ultraviolet. Using the code Cloudy, we modelled the intensities of the broad, double-peaked, low-ionization emission lines observed with the Hubble telescope, as emission from the atmosphere of the thin disk photoionized by the ionizing continuum emitted by the ADAF. Considering that we adopted a simplified prescription for the illumination of the accretion disk, we obtained a good agreement between the results of the simulation and the observations of the lines. It is the first time that such self-consistent modelling is done, in the sense that the parameters used in the modelling of the continuum and the flux of the double-peaked emission lines are the same as those derived in the modelling of double-peaked emission-line profiles in previous works. With the detailed continuum modelling of NGC1097 presented in this work added to the modelling of the temporal variability of the H® emission-line profile accomplished in previous works, we strengthen the scenario in which an ADAF + thin disk is responsible for the characteristic properties of active galactic nuclei (AGN) emitting \disklike" lines. Furthermore, this work furthers the hypothesis that the low luminosities of low-luminosity AGN are a consequence of the presence of radiatively ineffcient accretion flows in their nuclei.

NGC1097 é uma galáxia espiral a uma distância de 14.5 Mpc, com um núcleo LINER (do inglês Low-Ionization Nuclear Emission Region) de baixa luminosidade, a partir do qual foi descoberta em 1991 emissão larga de duplo pico nas linhas de Balmer do hidrogênio. O perfil destas linhas mostrou-se, em trabalho anterior, ser bem reproduzido por um modelo de disco de gás em rotação (disco de acreção), com velocidades de até 10000 km s¡1. Neste trabalho modelamos de forma autoconsistente o espectro contínuo de energia do núcleo LINER obtido a partir de observações recentes com o Telescópio de Raios X Chandra e com o Telescópio Espacial Hubble, bem como dados de arquivo na região dos comprimentos de onda de rádio e infravermelho. Além disso, utilizamos modelos de fotoionização para modelar o fluxo das linhas de emissão largas com duplo pico. O contínuo é modelado como tendo origem num escoamento acretivo de plasma ao redor do buraco negro central. A parte interna do escoamento mais próxima do buraco negro (R < 225RS, onde RS é o raio de Schwartzschild) é opticamente fina, dominada por advecção (chamada ADAF) e a parte externa (R > 225RS) consiste num disco de acreção geometricamente fino, opticamente espesso. A parte interna do escoamento (ADAF) é responsável por emissão que se estende da região espectral de rádio até raios X do contínuo observado, enquanto a parte externa (disco fino) é responsável por uma fraca emissão contínua no óptico - ultravioleta. Modelamos as intensidades das linhas de baixa ionização largas com duplo pico observadas com o telescópio Hubble como emissão da atmosfera do disco fino fotoionizada pelo contínuo ionizante emitido do ADAF, usando o programa Cloudy. Levando em conta que usamos uma prescrição simples da iluminação do disco, obtivemos uma boa concordância entre os resultados da simulação e as observações das linhas. É a primeira vez que é realizada uma modelagem autoconsistente como esta, no sentido de que os parâmetros utilizados na modelagem do contínuo e do fluxo das linhas de emissão são os mesmos derivados da modelagem dos perfis das linhas de emissão com duplo pico em trabalhos anteriores. Com a modelagem detalhada do contínuo de NGC1097 apresentada neste trabalho somada à modelagem da variabilidade temporal do perfil da linha H alfa realizada em trabalhos anteriores, fortalecemos o cenário ADAF + disco fino para o escoamento acretivo como origem das propriedades características dos AGNs emissores de linhas largas com duplo pico. Além disso, este trabalho reforça a hipótese de que as baixas luminosidades dos LLAGNs (Low-Luminosity Active Galactic Nuclei) são uma consequência da existência de escoamentos de baixa eficiência radiativa nos seus núcleos. / NGC1097 is a spiral galaxy at a distance of 14.5 Mpc which harbors a low-luminosity LINER (Low-Ionization Nuclear Emission Region) nucleus. In 1991 this galaxy displayed the abrupt appearance of broad double-peaked Balmer lines, and in subsequent work the double-peaked H® line profile was modelled as emission from a disk of rotating gas (accretion disk), with velocities up to 10000 km s¡1. In this work we model in a self-consistent way the continuum energy spectrum of the LINER nucleus of NGC1097, obtained through recent observations with the X-ray telescope Chandra and the Hubble Space Telescope, as well as data in the wavelength region of radio. Furthermore, we used a photoionization code to model the flux of the broad double-peaked emission lines. The continuum is modelled as being originated in an accretion flow of plasma around the central black hole. The inner part of the flow closer to the black hole (R < 225RS, where RS is the Schwartzschild radius) is optically thin, geometrically thick, advection- dominated (named ADAF) and the outer portion consists of a geometrically thick, optically thin accretion disk. The ADAF is responsible for continuum emission that extends from radio to X-rays, while the thin disk is responsible for a weak continuum emission in the optical to ultraviolet. Using the code Cloudy, we modelled the intensities of the broad, double-peaked, low-ionization emission lines observed with the Hubble telescope, as emission from the atmosphere of the thin disk photoionized by the ionizing continuum emitted by the ADAF. Considering that we adopted a simplified prescription for the illumination of the accretion disk, we obtained a good agreement between the results of the simulation and the observations of the lines. It is the first time that such self-consistent modelling is done, in the sense that the parameters used in the modelling of the continuum and the flux of the double-peaked emission lines are the same as those derived in the modelling of double-peaked emission-line profiles in previous works. With the detailed continuum modelling of NGC1097 presented in this work added to the modelling of the temporal variability of the H® emission-line profile accomplished in previous works, we strengthen the scenario in which an ADAF + thin disk is responsible for the characteristic properties of active galactic nuclei (AGN) emitting \disklike" lines. Furthermore, this work furthers the hypothesis that the low luminosities of low-luminosity AGN are a consequence of the presence of radiatively ineffcient accretion flows in their nuclei.

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro de Ciências Físicas e Matemáticas. Programa de Pós-graduação em Física / Made available in DSpace on 2012-10-21T12:32:11Z (GMT). No. of bitstreams: 1 245041.pdf: 5420386 bytes, checksum: e46eafc3b72d964147b63715f5d65852 (MD5) / V4140 Sgr é uma VC eclipsante ainda não classificada como nova-anã ou como polar. Relata-se a análise de séries temporais de fotometria CCD rápida do objeto, obtidas no LNA entre 1991 e 2001, nas bandas BVR. O sistema foi observado em declínio de uma erupção em 1992 e em erupção em 2001, o que confirma a sua classificação como uma nova-anã. Fez-se a revisão dos parâmetros orbitais do sistema. Aplica-se o Método de Mapeamento por Eclipse (MME) às curvas de luz medianas do objeto em quiescência e erupção, para obtenção dos mapas de distribuição superficial de brilho do disco de acréscimo. Estima-se uma distância de 600 ± 100 pc ao sistema. Determina-se os perfis radiais de temperatura do disco, que seguem bem a lei para discos opacos estacionários, T R-3/4. As temperaturas nas partes internas do disco, tanto em erupção quanto em quiescência, são sistematicamente maiores que a temperatura crítica Tcrit abaixo da qual o gás no disco deve estar para permitir erupções, segundo o modelo de instabilidade no disco. Apresenta um comportamento bastante peculiar em relação a outras VCs com período orbital similar (OY Car, HT Cas e Z Cha).

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro de Ciências Físicas e Matemáticas. Programa de Pós-Graduação em Física. / Made available in DSpace on 2012-10-22T02:52:12Z (GMT). No. of bitstreams: 1 221641.pdf: 2521432 bytes, checksum: 59e353353846b1075ec15644c6449f71 (MD5) / Apresentamos um estudo do espectro e da estrutura do disco de acréscimo da nova-anã IP Pegasi no declínio de uma erupção, com o auxílio da técnica de mapeamento por eclipse, a partir de dados de espectroscopia no ultravioleta de três eclipses. Os mapas de eclipse da primeira das monitorias apresentam evidências de braços espirais, 10 dias após o início da erupção. Nos espectros espacialmente resolvidos as linhas mais intensas aparecem em emissão em qualquer raio, sendo mais intensas nas partes internas do disco. O espectro da emissão do gas stream difere de forma notável do espectro do disco nas partes intermediárias e externas. As linhas aparecem em absorção em todos os raios e a emissão diferenciada evidencia o transbordamento do jorro de matéria. A largura à meia altura da linha de C IV é aproximadamente constante com o raio, em contraste com a dependência esperada para um gás em órbita Kepleriana, sugerindo sua origem em uma região verticalmente extensa. A componente não-eclipsada contribui com ~ 4% do fluxo na linha de C IV na primeira das monitorias, sendo desprezível nos outros comprimentos de onda. A distribuição radial de temperatura no disco nas três monitorias é mais plana que o esperado pelo modelo de discos opacos em estado estacionário. A dependência radial dos ângulos sólidos indica que o ângulo de abertura aumenta com o raio do disco.

In this thesis two related arguments are investigated: - The first stages of the process of massive star formation, investigating the physical conditions and -properties of massive clumps in different evolutionary stages, and their CO depletion; - The influence that high-mass stars have on the nearby material and on the activity of star formation. I characterise the gas and dust temperature, mass and density of a sample of massive clumps, and analyse the variation of these properties from quiescent clumps, without any sign of active star formation, to clumps likely hosting a zero-age main sequence star. I briefly discuss CO depletion and recent observations of several molecular species, tracers of Hot Cores and/or shocked gas, of a subsample of these clumps. The issue of CO depletion is addressed in more detail in a larger sample consisting of the brightest sources in the ATLASGAL survey: using a radiative tranfer code I investigate how the depletion changes from dark clouds to more evolved objects, and compare its evolution to what happens in the low-mass regime. Finally, I derive the physical properties of the molecular gas in the photon-dominated region adjacent to the HII region G353.2+0.9 in the vicinity of Pismis 24, a young, massive cluster, containing some of the most massive and hottest stars known in our Galaxy. I derive the IMF of the cluster and study the star formation activity in its surroundings. Much of the data analysis is done with a Bayesian approach. Therefore, a separate chapter is dedicated to the concepts of Bayesian statistics.

The open clusters (OC) are gravitationally bound systems of a few tens or hundreds of stars. In our Galaxy, the Milky Way, we know about 3000 open clusters, of very different ages in the range of a few millions years to about 9 Gyr. OCs are mainly located in the Galactic thin disc, with distances from the Galactic centre in the range 4-22 kpc and a height scale on the disc of about 200 pc. Their chemical properties trace those of the environment in which they formed and the metallicity is in the range -0.5<[Fe/H]<+0.5 dex. Through photometry and spectroscopy it is possible to study relatively easily the properties of the OCs and estimate their age, distance, and chemistry. For these reasons they are considered primary tracers of the chemical properties and chemical evolution of the Galactic disc. The main subject of this thesis is the comprehensive study of several OCs. The research embraces two different projects: the Bologna Open Cluster Chemical Evolution project (BOCCE) and the Gaia-ESO Survey. The first is a long-term programme, aiming at studying the chemical evolution of the Milky Way disc by means of a homogeneous sample of OCs. The latter is a large public spectroscopy survey, conducted with the high-resolution spectrograph FLAMES@VLT and targeting about 10^5 stars in different part of the Galaxy and 10^4 stars in about 100 OCs. The common ground between the two projects is the study of the properties of the OCs as tracers of the disc's characteristics. The impressive scientific outcome of the Gaia-ESO Survey and the unique framework of homogeneity of the BOCCE project can propose, especially once combined together, a much more accurate description of the properties of the OCs. In turn, this will give fundamental constraints for the interpretation of the properties of the Galactic disc.

The discovery of the Cosmic Microwave Background (CMB) radiation in 1965 is one of the fundamental milestones supporting the Big Bang theory. The CMB is one of the most important source of information in cosmology. The excellent accuracy of the recent CMB data of WMAP and Planck satellites confirmed the validity of the standard cosmological model and set a new challenge for the data analysis processes and their interpretation. In this thesis we deal with several aspects and useful tools of the data analysis. We focus on their optimization in order to have a complete exploitation of the Planck data and contribute to the final published results. The issues investigated are: the change of coordinates of CMB maps using the HEALPix package, the problem of the aliasing effect in the generation of low resolution maps, the comparison of the Angular Power Spectrum (APS) extraction performances of the optimal QML method, implemented in the code called BolPol, and the pseudo-Cl method, implemented in Cromaster. The QML method has been then applied to the Planck data at large angular scales to extract the CMB APS. The same method has been applied also to analyze the TT parity and the Low Variance anomalies in the Planck maps, showing a consistent deviation from the standard cosmological model, the possible origins for this results have been discussed. The Cromaster code instead has been applied to the 408 MHz and 1.42 GHz surveys focusing on the analysis of the APS of selected regions of the synchrotron emission. The new generation of CMB experiments will be dedicated to polarization measurements, for which are necessary high accuracy devices for separating the polarizations. Here a new technology, called Photonic Crystals, is exploited to develop a new polarization splitter device and its performances are compared to the devices used nowadays.

The diameters of traditional dish concentrators can reach several tens of meters, the construction of monolithic mirrors being difficult at these scales: cheap flat reflecting facets mounted on a common frame generally reproduce a paraboloidal surface. When a standard imaging mirror is coupled with a PV dense array, problems arise since the solar image focused is intrinsically circular. Moreover, the corresponding irradiance distribution is bell-shaped in contrast with the requirement of having all the cells under the same illumination. Mismatch losses occur when interconnected cells experience different conditions, in particular in series connections. In this PhD Thesis, we aim at solving these issues by a multidisciplinary approach, exploiting optical concepts and applications developed specifically for astronomical use, where the improvement of the image quality is a very important issue. The strategy we propose is to boost the spot uniformity acting uniquely on the primary reflector and avoiding the big mirrors segmentation into numerous smaller elements that need to be accurately mounted and aligned. In the proposed method, the shape of the mirrors is analytically described by the Zernike polynomials and its optimization is numerically obtained to give a non-imaging optics able to produce a quasi-square spot, spatially uniform and with prescribed concentration level. The freeform primary optics leads to a substantial gain in efficiency without secondary optics. Simple electrical schemes for the receiver are also required. The concept has been investigated theoretically modeling an example of CPV dense array application, including the development of non-optical aspects as the design of the detector and of the supporting mechanics. For the method proposed and the specific CPV system described, a patent application has been filed in Italy with the number TO2014A000016. The patent has been developed thanks to the collaboration between the University of Bologna and INAF (National Institute for Astrophysics).

Quasars and AGN play an important role in many aspects of the modern cosmology. Of particular interest is the issue of the interplay between AGN activity and formation and evolution of galaxies and structures. Studies on nearby galaxies revealed that most (and possibly all) galaxy nuclei contain a super-massive black hole (SMBH) and that between a third and half of them are showing some evidence of activity (Kormendy and Richstone, 1995). The discovery of a tight relation between black holes mass and velocity dispersion of their host galaxy suggests that the evolution of the growth of SMBH and their host galaxy are linked together. In this context, studying the evolution of AGN, through the luminosity function (LF), is fundamental to constrain the theories of galaxy and SMBH formation and evolution. Recently, many theories have been developed to describe physical processes possibly responsible of a common formation scenario for galaxies and their central black hole (Volonteri et al., 2003; Springel et al., 2005a; Vittorini et al., 2005; Hopkins et al., 2006a) and an increasing number of observations in different bands are focused on collecting larger and larger quasar samples. Many issues remain however not yet fully understood. In the context of the VVDS (VIMOS-VLT Deep Survey), we collected and studied an unbiased sample of spectroscopically selected faint type-1 AGN with a unique and straightforward selection function. Indeed, the VVDS is a large, purely magnitude limited spectroscopic survey of faint objects, free of any morphological and/or color preselection. We studied the statistical properties of this sample and its evolution up to redshift z 4. Because of the contamination of the AGN light by their host galaxies at the faint magnitudes explored by our sample, we observed that a significant fraction of AGN in our sample would be missed by the UV excess and morphological criteria usually adopted for the pre-selection of optical QSO candidates. If not properly taken into account, this failure in selecting particular sub-classes of AGN could, in principle, affect some of the conclusions drawn from samples of AGN based on these selection criteria. The absence of any pre-selection in the VVDS leads us to have a very complete sample of AGN, including also objects with unusual colors and continuum shape. The VVDS AGN sample shows in fact redder colors than those expected by comparing it, for example, with the color track derived from the SDSS composite spectrum. In particular, the faintest objects have on average redder colors than the brightest ones. This can be attributed to both a large fraction of dust-reddened objects and a significant contamination from the host galaxy. We have tested these possibilities by examining the global spectral energy distribution of each object using, in addition to the U, B, V, R and I-band magnitudes, also the UV-Galex and the IR-Spitzer bands, and fitting it with a combination of AGN and galaxy emission, allowing also for the possibility of extinction of the AGN flux. We found that for 44% of our objects the contamination from the host galaxy is not negligible and this fraction decreases to 21% if we restrict the analysis to a bright subsample (M1450 <-22.15). Our estimated integral surface density at IAB < 24.0 is 500 AGN per square degree, which represents the highest surface density of a spectroscopically confirmed sample of optically selected AGN. We derived the luminosity function in B-band for 1.0 < z < 3.6 using the 1/Vmax estimator. Our data, more than one magnitude fainter than previous optical surveys, allow us to constrain the faint part of the luminosity function up to high redshift. A comparison of our data with the 2dF sample at low redshift (1 < z < 2.1) shows that the VDDS data can not be well fitted with the pure luminosity evolution (PLE) models derived by previous optically selected samples. Qualitatively, this appears to be due to the fact that our data suggest the presence of an excess of faint objects at low redshift (1.0 < z < 1.5) with respect to these models. By combining our faint VVDS sample with the large sample of bright AGN extracted from the SDSS DR3 (Richards et al., 2006b) and testing a number of different evolutionary models, we find that the model which better represents the combined luminosity functions, over a wide range of redshift and luminosity, is a luminosity dependent density evolution (LDDE) model, similar to those derived from the major Xsurveys. Such a parameterization allows the redshift of the AGN density peak to change as a function of luminosity, thus fitting the excess of faint AGN that we find at 1.0 < z < 1.5. On the basis of this model we find, for the first time from the analysis of optically selected samples, that the peak of the AGN space density shifts significantly towards lower redshift going to lower luminosity objects. The position of this peak moves from z 2.0 for MB <-26.0 to z 0.65 for -22< MB <-20. This result, already found in a number of X-ray selected samples of AGN, is consistent with a scenario of “AGN cosmic downsizing”, in which the density of more luminous AGN, possibly associated to more massive black holes, peaks earlier in the history of the Universe (i.e. at higher redshift), than that of low luminosity ones, which reaches its maximum later (i.e. at lower redshift). This behavior has since long been claimed to be present in elliptical galaxies and it is not easy to reproduce it in the hierarchical cosmogonic scenario, where more massive Dark Matter Halos (DMH) form on average later by merging of less massive halos.